In the process of manufacturing a gear portion for an automobile, for example, a leg portion of a carrier cover and a disk-shaped member in the gear portion are often mechanically bonded to each other by electrical press-fit bonding called arc welding or ring mash (trademark) welding. In the case of arc welding, the leg portion of the carrier cover is fitted into a welding hole formed in the disk-shaped member and the entire circumference or a part of the fitted part is welded by arc welding. However, in the case of arc welding, because the period of time for which thermal energy is injected into the welding target part is much longer than that in a capacitor stored-energy welding method in which welding is carried out by discharging energy stored in a capacitor within a short period of time, thermal strain occurs in a shaft member of the carrier cover and the disk-shaped member as welding target objects, resulting in a lower dimensional accuracy. Another drawback is that the welded part between the metal materials and its vicinity undergo significant thermal deterioration or color change.
To solve such problems, an electrical press-fit bonding method has been proposed including positioning several divided leg portions of a carrier cover at bonding holes formed in a disk-shaped member, applying a bonding current to the leg portions under a pressing force to cause end parts of the leg portions of the carrier cover to undergo plastic flow and enter the bonding holes to solid-phase bond them to the disk-shaped member (see Patent Document 1, for example). However, in the bonding method and structure disclosed in Patent Document 1, the bonding holes are formed at the center of the disk-shaped member and the divided leg portions of the carrier cover are press-fit into the welding holes at a plurality of positions. Thus, when the welding target objects are employed in a mechanism in which large torque is applied between the disk-shaped member and the leg portions of the carrier cover, a large rotative force, in other words, torque is applied in such a direction as to shear the solid-phase bonded parts between the disk-shaped member and the leg portions of the carrier cover, in other words, in a direction parallel to the bonded surfaces between the disk-shaped member and the leg portions of the carrier cover.
In the case of the above-mentioned solid-phase bonding, the surfaces of the welding target objects are simply bonded in a plastic flow state. Thus, the bonding area is small and its mechanical strength is not high because the bonding depth of the bonded portion is shallow compared to the case of resistance welding where the surfaces are melted to form a nugget. In a bonded article obtained by such a bonding method, the mechanical strength in such a direction as to shear the bonded portion between the disk-shaped member and the several divided leg portions of the carrier cover cannot be high. Thus, the torque that can be applied to the bonded article is significantly limited. To remove such a drawback, a bonding method is disclosed in which leg portions of a carrier cover are press-fit bonded to the short sides of rectangular bonding holes of a plate-like bonding target member and not to the long sides thereof (see Patent Document 2, for example). In this bonding method, because the bonding current can be concentrated at the short side parts of the both members, the bonding depth of the bonded portion at the short side parts can be large to increase the bonding strength. The problem, however, is that higher strength cannot be obtained when the short side parts are short, in other words, the plate thickness at the short side parts is small.